rustdoc-json: Include GenericParamDefKind::Type::synthetic in JSON

The rustdoc JSON for

```
pub fn f(_: impl Clone) {}
```

will effectively be

```
pub fn f<impl Clone: Clone>(_: impl Clone)
```

where a synthetic generic parameter called `impl Clone` with generic
trait bound `Clone` is added to the function declaration.

The generated HTML filters out these generic parameters by doing
`self.params.iter().filter(|p| !p.is_synthetic_type_param())`, because
the synthetic generic parameter is not of interest to regular users.

For the same reason, we should expose whether or not a generic parameter
is synthetic or not also in the rustdoc JSON, so that rustdoc JSON
clients can also have the option to hide synthetic generic parameters.

src/rustdoc-json-types/lib.rs

@@ -9,7 +9,7 @@ use std::path::PathBuf;
 use serde::{Deserialize, Serialize};
 
 /// rustdoc format-version.
-pub const FORMAT_VERSION: u32 = 12;
+pub const FORMAT_VERSION: u32 = 13;
 
 /// A `Crate` is the root of the emitted JSON blob. It contains all type/documentation information
 /// about the language items in the local crate, as well as info about external items to allow
@@ -346,9 +346,41 @@ pub struct GenericParamDef {
 #[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]
 #[serde(rename_all = "snake_case")]
 pub enum GenericParamDefKind {
-    Lifetime { outlives: Vec<String> },
-    Type { bounds: Vec<GenericBound>, default: Option<Type> },
-    Const { ty: Type, default: Option<String> },
+    Lifetime {
+        outlives: Vec<String>,
+    },
+    Type {
+        bounds: Vec<GenericBound>,
+        default: Option<Type>,
+        /// This is normally `false`, which means that this generic parameter is
+        /// declared in the Rust source text.
+        ///
+        /// If it is `true`, this generic parameter has been introduced by the
+        /// compiler behind the scenes.
+        ///
+        /// # Example
+        ///
+        /// Consider
+        ///
+        /// ```ignore (pseudo-rust)
+        /// pub fn f(_: impl Trait) {}
+        /// ```
+        ///
+        /// The compiler will transform this behind the scenes to
+        ///
+        /// ```ignore (pseudo-rust)
+        /// pub fn f<impl Trait: Trait>(_: impl Trait) {}
+        /// ```
+        ///
+        /// In this example, the generic parameter named `impl Trait` (and which
+        /// is bound by `Trait`) is synthetic, because it was not originally in
+        /// the Rust source text.
+        synthetic: bool,
+    },
+    Const {
+        ty: Type,
+        default: Option<String>,
+    },
 }
 
 #[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]